Previous topic

Next topic

23.3. Creating Form Elements Using Zend_Form_Element

A form is made of elements that typically correspond to HTML form
input. Zend_Form_Element encapsulates single form elements, with the
following areas of responsibility:

validation (is submitted data valid?)

capturing of validation error codes and messages

filtering (how is the element escaped or normalized prior to
validation and/or for output?)

rendering (how is the element displayed?)

metadata and attributes (what information further qualifies the element?)

The base class, Zend_Form_Element, has reasonable defaults
for many cases, but it is best to extend the class for commonly used
special purpose elements. Additionally, Zend Framework ships with a
number of standard XHTML elements; you can read about them in the Standard Elements
chapter.

23.3.1. Plugin Loaders

Zend_Form_Element makes use of Zend_Loader_PluginLoader
to allow developers to specify locations of alternate validators,
filters, and decorators. Each has its own plugin loader associated
with it, and general accessors are used to retrieve and modify
each.

The following loader types are used with the various plugin loader
methods: 'validate', 'filter', and 'decorator'. The type names are
case insensitive.

The methods used to interact with plugin loaders are as follows:

setPluginLoader($loader, $type):
$loader is the plugin loader object itself, while
$type is one of the types specified above. This
sets the plugin loader for the given type to the newly
specified loader object.

addPrefixPath($prefix, $path, $type = null): adds
a prefix/path association to the loader specified by
$type. If $type is null, it will
attempt to add the path to all loaders, by appending the prefix
with each of "_Validate", "_Filter", and "_Decorator"; and
appending the path with "Validate/", "Filter/", and
"Decorator/". If you have all your extra form element classes
under a common hierarchy, this is a convenience method for
setting the base prefix for them.

addPrefixPaths(array $spec): allows you to add
many paths at once to one or more plugin loaders. It expects
each array item to be an array with the keys 'path', 'prefix',
and 'type'.

Custom validators, filters, and decorators are an easy way to share
functionality between forms and to encapsulate custom functionality.

Example 23.1. Custom Label

One common use case for plugins is to provide replacements for
standard classes. For instance, if you want to provide a
different implementation of the 'Label' decorator -- for
instance, to always append a colon -- you could create your own
'Label' decorator with your own class prefix, and then add it to
your prefix path.

Let's start with a custom Label decorator. We'll give it the
class prefix "My_Decorator", and the class itself will be in the
file "My/Decorator/Label.php".

After it added as in the example above, the 'My/Decorator/' path will be searched first to see if the
decorator exists there when you add a decorator. As a result, 'My_Decorator_Label' will
now be used when the 'Label' decorator is requested.

23.3.2. Filters

It's often useful and/or necessary to perform some normalization on
input prior to validation. For example, you may want to strip out
all HTML, but run your validations on what remains to ensure the
submission is valid. Or you may want to trim empty space surrounding input so that a StringLength validator will use the correct length of the input without counting leading or trailing whitespace characters.
These operations may be performed using
Zend_Filter. Zend_Form_Element has
support for filter chains, allowing you to specify multiple,
sequential filters. Filtering happens both during
validation and when you retrieve the element value via
getValue():

$filtered = $element->getValue();

Filters may be added to the chain in two ways:

passing in a concrete filter instance

providing a filter name – either a short name or fully
qualified class name

Short names are typically the filter name minus the prefix. In the
default case, this will mean minus the 'Zend_Filter_' prefix.
The first letter can be upper-cased or lower-cased.

Using Custom Filter Classes

If you have your own set of filter classes, you can tell
Zend_Form_Element about these using
addPrefixPath(). For instance, if you have
filters under the 'My_Filter' prefix, you can tell
Zend_Form_Element about this as follows:

$element->addPrefixPath('My_Filter', 'My/Filter/', 'filter');

(Recall that the third argument indicates which plugin loader
on which to perform the action.)

If at any time you need the unfiltered value, use the
getUnfilteredValue() method:

23.3.3. Validators

If you subscribe to the security mantra of "filter input, escape
output," you'll should use validator to filter input submitted with your form.
In Zend_Form, each element includes its own validator
chain, consisting of Zend_Validate_* validators.

Validators may be added to the chain in two ways:

passing in a concrete validator instance

providing a validator name – either a short name or fully
qualified class name

Short names are typically the validator name minus the prefix. In
the default case, this will mean minus the 'Zend_Validate_' prefix.
As is the case with filters, the first letter can be upper-cased or lower-cased.

Using Custom Validator Classes

If you have your own set of validator classes, you can tell
Zend_Form_Element about these using
addPrefixPath(). For instance, if you have
validators under the 'My_Validator' prefix, you can tell
Zend_Form_Element about this as follows:

$element->addPrefixPath('My_Validator', 'My/Validator/', 'validate');

(Recall that the third argument indicates which plugin loader
on which to perform the action.)

If failing a particular validation should prevent later validators
from firing, pass boolean true as the second parameter:

$element->addValidator('alnum', true);

If you are using a string name to add a validator, and the
validator class accepts arguments to the constructor, you may pass
these to the third parameter of addValidator() as an
array:

$element->addValidator('StringLength', false, array(6, 20));

Arguments passed in this way should be in the order in which they
are defined in the constructor. The above example will instantiate
the Zend_Validate_StringLenth class with its
$min and $max parameters:

$validator = new Zend_Validate_StringLength(6, 20);

Providing Custom Validator Error Messages

Some developers may wish to provide custom error messages for a
validator. The $options argument of the Zend_Form_Element::addValidator() method
allows you to do so by providing
the key 'messages' and mapping it to an array of key/value pairs
for setting the message templates. You will need to know the
error codes of the various validation error types for the
particular validator.

A better option is to use a Zend_Translate_Adapter
with your form. Error codes are automatically passed to the
adapter by the default Errors decorator; you can then specify
your own error message strings by setting up translations for
the various error codes of your validators.

You can also set many validators at once, using
addValidators(). The basic usage is to pass an array
of arrays, with each array containing 1 to 3 values, matching the
constructor of addValidator():

Notice that every item has a key, whether or not it needs one; this
is a limitation of using configuration files -- but it also helps
make explicit what the arguments are for. Just remember that any
validator options must be specified in order.

To validate an element, pass the value to
isValid():

if ($element->isValid($value)) {
// valid
} else {
// invalid
}

Validation Operates On Filtered Values

Zend_Form_Element::isValid() filters values through
the provided filter chain prior to validation. See the Filters
section for more information.

Validation Context

Zend_Form_Element::isValid() supports an
additional argument, $context.
Zend_Form::isValid() passes the entire array of
data being processed to $context when validating a
form, and Zend_Form_Element::isValid(), in turn,
passes it to each validator. This means you can write
validators that are aware of data passed to other form
elements. As an example, consider a standard registration form
that has fields for both password and a password confirmation;
one validation would be that the two fields match. Such a
validator might look like the following:

Validators are processed in order. Each validator is processed,
unless a validator created with a true
breakChainOnFailure value fails its validation. Be
sure to specify your validators in a reasonable order.

After a failed validation, you can retrieve the error codes and
messages from the validator chain:

In addition to validators, you can specify that an element is
required, using setRequired(true). By default, this
flag is false, meaning that your validator chain will be skipped if
no value is passed to isValid(). You can modify this
behavior in a number of ways:

By default, when an element is required, a flag,
'allowEmpty', is also true. This means that if a value
evaluating to empty is passed to isValid(), the
validators will be skipped. You can toggle this flag using
the accessor setAllowEmpty($flag); when the
flag is false and a value is passed, the validators
will still run.

By default, if an element is required but does not contain
a 'NotEmpty' validator, isValid() will add one
to the top of the stack, with the
breakChainOnFailure flag set. This behavior lends
required flag semantic meaning: if no value is passed,
we immediately invalidate the submission and notify the
user, and prevent other validators from running on what we
already know is invalid data.

If you do not want this behavior, you can turn it off by
passing a false value to
setAutoInsertNotEmptyValidator($flag); this
will prevent isValid() from placing the
'NotEmpty' validator in the validator chain.

Zend_Form_Element implements
Zend_Validate_Interface, meaning an element may
also be used as a validator in other, non-form related
validation chains.

Methods associated with validation include:

setRequired($flag) and
isRequired() allow you to set and retrieve the
status of the 'required' flag. When set to boolean true, this
flag requires that the element be in the data processed by
Zend_Form.

setAllowEmpty($flag) and
getAllowEmpty() allow you to modify the
behaviour of optional elements (i.e., elements where the
required flag is false). When the 'allow empty' flag is
true, empty values will not be passed to the validator
chain.

setAutoInsertNotEmptyValidator($flag) allows
you to specify whether or not a 'NotEmpty' validator will be
prepended to the validator chain when the element is
required. By default, this flag is true.

23.3.3.1. Custom Error Messages

At times, you may want to specify one or more specific error
messages to use instead of the error messages generated by the
validators attached to your element. Additionally, at times you
may want to mark the element invalid yourself. As of 1.6.0, this
functionality is possible via the following methods.

addErrorMessage($message): add an error message
to display on form validation errors. You may call this more
than once, and new messages are appended to the stack.

getErrorMessages(): retrieve the list of
custom error messages that have been defined.

clearErrorMessages(): remove all custom error
messages that have been defined.

markAsError(): mark the element as having
failed validation.

hasErrors(): determine whether the element has
either failed validation or been marked as invalid.

addError($message): add a message to the custom
error messages stack and flag the element as invalid.

addErrors(array $messages): add several
messages to the custom error messages stack and flag the
element as invalid.

setErrors(array $messages): overwrite the
custom error messages stack with the provided messages and
flag the element as invalid.

All errors set in this fashion may be translated. Additionally,
you may insert the placeholder "%value%" to represent the
element value; this current element value will be substituted
when the error messages are retrieved.

23.3.4. Decorators

One particular pain point for many web developers is the creation
of the XHTML forms themselves. For each element, the developer
needs to create markup for the element itself (typically a label)
and special markup for displaying
validation error messages. The more elements on the page, the less
trivial this task becomes.

Zend_Form_Element tries to solve this issue through
the use of "decorators". Decorators are simply classes that have
access to the element and a method for rendering content. For more
information on how decorators work, please see the section on Zend_Form_Decorator.

The default decorators used by Zend_Form_Element are:

ViewHelper: specifies a view helper to use
to render the element. The 'helper' element attribute can be
used to specify which view helper to use. By default,
Zend_Form_Element specifies the 'formText' view
helper, but individual subclasses specify different helpers.

Errors: appends error messages to the
element using Zend_View_Helper_FormErrors. If none are present,
nothing is appended.

Description: appends the element
description. If none is present, nothing is appended. By
default, the description is rendered in a <p> tag with a
class of 'description'.

HtmlTag: wraps the element and errors in
an HTML <dd> tag.

Label: prepends a label to the element
using Zend_View_Helper_FormLabel, and wraps it in a <dt>
tag. If no label is provided, just the definition term tag is
rendered.

Default Decorators Do Not Need to Be Loaded

By default, the default decorators are loaded during object
initialization. You can disable this by passing the
'disableLoadDefaultDecorators' option to the constructor:

This option may be mixed with any other options you pass,
both as array options or in a Zend_Config object.

Since the order in which decorators are registered matters- the first
decorator registered is executed first- you will need to make
sure you register your decorators in an appropriate order, or
ensure that you set the placement options in a sane fashion. To
give an example, here is the code that registers the default
decorators:

The initial content is created by the 'ViewHelper' decorator, which
creates the form element itself. Next, the 'Errors' decorator
fetches error messages from the element, and, if any are present,
passes them to the 'FormErrors' view helper to render. If a
description is present, the 'Description' decorator will append a
paragraph of class 'description' containing the descriptive text to
the aggregated content. The next decorator, 'HtmlTag', wraps the
element, errors, and description in an HTML <dd> tag.
Finally, the last decorator, 'label', retrieves the element's label
and passes it to the 'FormLabel' view helper, wrapping it in an HTML
<dt> tag; the value is prepended to the content by default.
The resulting output looks basically like this:

Internally, Zend_Form_Element uses a decorator's
class as the lookup mechanism when retrieving decorators. As a
result, you cannot register multiple decorators of the same
type; subsequent decorators will simply overwrite those that
existed before.

To get around this, you can use aliases.
Instead of passing a decorator or decorator name as the first
argument to addDecorator(), pass an array with a
single element, with the alias pointing to the decorator object
or name:

Zend_Form_Element also uses overloading to allow rendering
specific decorators. __call() will intercept methods
that lead with the text 'render' and use the remainder of the method
name to lookup a decorator; if found, it will then render that
single decorator. Any arguments passed to the
method call will be used as content to pass to the decorator's
render() method. As an example:

23.3.5. Metadata and Attributes

label: the element label. Uses the
setLabel() and getLabel() accessors.

order: the index at which an element
should appear in the form. Uses the setOrder() and
getOrder() accessors.

value: the current element value. Uses the
setValue() and getValue() accessors.

description: a description of the element;
often used to provide tooltip or javascript contextual hinting
describing the purpose of the element. Uses the
setDescription() and getDescription()
accessors.

required: flag indicating whether or not
the element is required when performing form validation. Uses
the setRequired() and getRequired()
accessors. This flag is false by default.

allowEmpty: flag indicating whether or not
a non-required (optional) element should attempt to validate
empty values. If it is set to true and the required flag is false, empty
values are not passed to the validator chain and are presumed true.
Uses the setAllowEmpty() and getAllowEmpty()
accessors. This flag is true by default.

autoInsertNotEmptyValidator: flag
indicating whether or not to insert a 'NotEmpty' validator when
the element is required. By default, this flag is true. Set the
flag with setAutoInsertNotEmptyValidator($flag) and
determine the value with
autoInsertNotEmptyValidator().

Form elements may require additional metadata. For XHTML form
elements, for instance, you may want to specify attributes such as
the class or id. To facilitate this are a set of accessors:

setAttrib($name, $value): add an attribute

setAttribs(array $attribs): like
addAttribs(), but overwrites

getAttrib($name): retrieve a single
attribute value

getAttribs(): retrieve all attributes as
key/value pairs

Most of the time, however, you can simply access them as object
properties, as Zend_Form_Element utilizes overloading
to facilitate access to them:

23.3.8. Configuration

Zend_Form_Element's constructor accepts either an
array of options or a Zend_Config object containing
options, and it can also be configured using either
setOptions() or setConfig(). Generally
speaking, keys are named as follows:

If 'set' + key refers to a Zend_Form_Element
method, then the value provided will be passed to that method.

Otherwise, the value will be used to set an attribute.

Exceptions to the rule include the following:

prefixPath will be passed to
addPrefixPaths()

The following setters cannot be set in this way:

setAttrib (though
setAttribswill
work)

setConfig

setOptions

setPluginLoader

setTranslator

setView

As an example, here is a config file that passes configuration for
every type of configurable data:

23.3.9. Custom Elements

You can create your own custom elements by simply extending the
Zend_Form_Element class. Common reasons to do so
include:

Elements that share common validators and/or filters

Elements that have custom decorator functionality

There are two methods typically used to extend an element:
init(), which can be used to add custom initialization
logic to your element, and loadDefaultDecorators(),
which can be used to set a list of default decorators used by your
element.

As an example, let's say that all text elements in a form you are
creating need to be filtered with StringTrim,
validated with a common regular expression, and that you want to
use a custom decorator you've created for displaying them,
'My_Decorator_TextItem'. In addition, you have a number of standard
attributes, including 'size', 'maxLength', and 'class' you wish to
specify. You could define an element to accomplish this as follows:

The 'foo' element will now be of type My_Element_Text,
and exhibit the behaviour you've outlined.

Another method you may want to override when extending
Zend_Form_Element is the
loadDefaultDecorators() method. This method
conditionally loads a set of default decorators for your element;
you may wish to substitute your own decorators in your extending
class: